Some fruits are turning dark purple and shriveled prior to harvest
Certified GKH Gardening Expert
This is normal for a plant that has more fruit than it can support at the time. It is not, usually, a cause for concern. I would fertilize with a fertilizer specific to blueberries. They contain ammonium sulfate, rather than nitrates. This will be better suited for them. Also, dolomitic lime, and iron sulfate will help with conditioning the soil, and sweetening the fruit.
This article will give you more information on the care of blueberry plants: https://www.gardeningknowhow.com/edible/fruits/blueberries/growing-blueberry.htm